SETI@home is not very GPU intensive. Their GPU application also utilizes CPU for some calculations, so a lot of data is shuffled between the CPU and the GPU which makes a 100% GPU workload unlikely.
There are BOINC applications which could easily outmatch the power consumption of LTC hashing, for example Primegrid GFN-22 (looking for very large prime numbers). But if shorter GFN tasks are failing on your GPUs, GFN-22 will more than likely fail too. It could be that those tasks are failing precisely because they are hard on the GPU.
It looks that PrimeGrid works perfectly with the old GeForce GT630 in Ubuntu 16.04. I will let this one working on PrimeGrid for a while, and use the time to find a solution also for the HD7990.Thanks, @vortac!
Many thanks for your support!